In recent years, in the production of bases such as printed circuit boards which are used in electrical devices, especially personal computers, plating methods known as “through-hole plating” or “via filling plating” have been used. Copper electroplating has rapid rates of deposition of plating films in the range of 10-50 μm/hr, and therefore is expected to find application in through-hole and via plating. However, if copper is deposited on the whole inner surface of a via, the inner part of the via is filled with copper without leaving a gap; therefore, it is necessary to make the deposition rate near the bottom surface of the via faster than the rate in the opening part. If the deposition rate near the bottom is the same as or slower than the rate at the opening part, then either the via will not be filled, or the opening part will be plugged up before the copper plating filling of the inner part of the via is completed, and a gap will be left in the inner part. In either case, the product will not be suitable for use. Moreover, in through-hole plating, the throwing power into the through-hole must be good.
Up to now, in order to promote the deposition rates near the bottom surface in the via and the surface of the through-hole wall, copper electroplating baths containing specific compounds with sulfur atoms in them have been used, and the electrolysis conditions generally used direct-current electroplating, using soluble anodes such as phosphorus-containing copper anodes. However, with such a method, good via filling ability is shown immediately after the bath is made up, but over time the copper electroplating bath becomes unstable, causing lumps to form in the formation of the copper electroplating layer after a specific time has passed since the bath was made up. Various problems arise, for example, the appearance of the plating becomes worse, the filling of the vias becomes unstable, etc. Moreover, in through-hole plating, the throwing power decreases and the reliability with respect to thermal shocks decreases.
In order to solve these problems, Japanese unexamined patent application No. 2002-249891 discloses a copper electroplating solution which contains specific compounds containing sulfur atoms and thiol reactive compounds. Examples of the thiol reactive compounds are carboxylic acids, including aliphatic and alicyclic compounds, aromatic compounds, and heterocyclic compounds, peroxo acids, aldehydes and ketones, and hydrogen peroxide. It is stated in the working examples that formaldehyde improves the filling ability. In recent years, however, efforts have been made to look for other useful compounds with via filling ability improvement performances to replace formaldehyde, taking into consideration its effects on the environment and the human body, the fact that its flash point is low (66° C.), etc.